Revisit compare in map and set

Posted by chunyang on May 7, 2020
TAGS: #cpp

map and set are two common associative containers. By default, common data types can be directly used as the key and their comparing method is std::less. On occassions, we need to store user-defined type as the key. We need to define the comparing method for the type.

Main course

There are two ways that we can set the comparing functions.

  • A struct class which defines operator()
  • A lambda function
#include <iostream>
#include <vector>
#include <map>
#include <string>
#include <functional>

using namespace std;
class Solution {

    struct Cmp {

        bool operator()(const string& l, const string& r) const {
            string ll = l;
            string rr = r;
            sort(begin(ll), end(ll));
            sort(begin(rr), end(rr));
            cout << "ll: " << ll <<endl;
            cout << "rr: " << rr << endl;
            return ll.compare(rr) < 0;
        }
    };
public:
    vector<vector<string>> groupAnagrams(vector<string>& strs) {
        const function<bool(const string&, const string&)> cmp = [](const string& l, const string& r) {
            string ll = l;
            string rr = r;
            sort(begin(ll), end(ll));
            sort(begin(rr), end(rr));
            cout << "ll: " << ll <<endl;
            cout << "rr: " << rr << endl;
            return ll.compare(rr) < 0;
        };
        /* Method 1: */
        map<string, vector<string>, decltype(cmp)> m(cmp);

        /* Method 2: */
        map<string, vector<string>, Cmp> m1;
        vector<vector<string>> result;
        for (string& s: strs) {
            m[s].push_back(s);
        }
        for (auto& x : m) {
            result.push_back(x.second);
        }
        return result;
    }
};


int main() {

    vector<string> s{"hi", "ih"};
    Solution sol;
    sol.groupAnagrams(s);

    return 0;
}
  • If we choose to use a struct type, operator() must be decorated by const (not the argument)
    • Otherwise you will get a compiliation error
  • If we we choose to use a lambda function, we have to pass the function to the constructor
    • Otherwise you will get a wierd error: bad_function_call

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